Bacteriophages drive strain diversification in a marine Flavobacterium: implications for phage resistance and physiological properties

Abstract

Genetic, structural and physiological differences between strains of the marine bacterium Cellulophaga baltica MM#3 (Flavobacteriaceae) developing in response to the activity of two virulent bacteriophages, Phi S(M) and Phi S(T), was investigated during 3 weeks incubation in chemostat cultures. A distinct strain succession towards increased phage resistance and a diversification of the metabolic properties was observed. During the incubation the bacterial population diversified from a single strain, which was sensitive to 24 tested Cellulophaga phages, into a multistrain and multiresistant population, where the dominant strains had lost susceptibility to up to 22 of the tested phages. By the end of the experiment the cultures reached a quasi steady state dominated by Phi S(T)-resistant and Phi S(M) + Phi S(T)-resistant strains coexisting with small populations of phage-sensitive strains sustaining both phages at densities of > 10(6) plaque forming units (pfu) ml(-1). Loss of susceptibility to phage infection was associated with a reduction in the strains' ability to metabolize various carbon sources as demonstrated by BIOLOG assays. This suggested a cost of resistance in terms of reduced physiological capacity. However, there was no direct correlation between the degree of resistance and the loss of metabolic properties, suggesting either the occurrence of compensatory mutations in successful strains or that the cost of resistance in some strains was associated with properties not resolved by the BIOLOG assay. The study represents the first direct demonstration of phage-driven generation of functional diversity within a marine bacterial host population with significant implications for both phage susceptibility and physiological properties. We propose, therefore, that phage-mediated selection for resistant strains contributes significantly to the extensive microdiversity observed within specific bacterial species in marine environments.

Viral and microbial community dynamics in four aquatic environments

“…Rapid evolution of resistant microbial strains and their viral predators is part of the perpetual arms race referred to as Red Queen dynamics. These evolutionary changes are routinely observed in chemostat studies (Bohannan and Lenski, 2002;Lennon and Martiny, 2008;Middelboe et al, 2009) and in environmental surveys (Vos et al, 2009). Our results do not distinguish between variation due to selection acting on pre-existing diversity or de novo generation of mutants.…”

Rapid diversification of coevolving marine Synechococcus and a virus

“…Selection for viral resistance is known to alter nutrient acquisition rates in marine bacteria (27,41), and in laboratory experiments, the fraction of Synechococcus cells that are resistant to co-occurring cyanophages influences nutrient turnover rates (46). Taken together, this evidence suggests that rapid coevolutionary processes may influence how viruses mediate nutrient cycling in the ocean.…”

Global occurrence and heterogeneity of the Roseobacter-clade species Ruegeria mobilis

“…Phages have previously been shown to be important drivers of bacterial diversification at the strain level by selecting for phageresistant strains, thus leading to a local arms race and co-evolution between phage and host populations (Middelboe et al, 2009). Hence, the versatility within the species is prompted by the exchange of genetic material as it has been proposed for the Roseobacter clade (Newton et al, 2010) and is not associated with any oceanographic parameters, which has been confirmed by the analysis of the TARA data (Supplementary Table S6).…”